1. Field of the Invention
The subject relay system is generally directed to a system for selectively coupling an energy source to an electrical load. More specifically, the subject relay system is directed to an electro-optically driven solid state relay system having a sufficiently accelerated switching response time to be adaptable for use in many systems wherein reliability and safety concerns are paramount.
One of numerous examples of such systems is found in a defibrillation or other medical electronic support system serving to restore or maintain a patient""s vital physiological functions. A defibrillation system is one that operates to apply a sharp, high-voltage pulse to one or more chambers of the patient""s heart when fibrillation (rapid, irregular, and disorganized contraction of cardiac muscle) occurs. A defibrillation system thus serves to xe2x80x98shockxe2x80x99 an ailing heart back into a rhythmic beating action, thereby restoring the synchronism necessary for it to serve its proper function.
If not sufficiently remedied in this manner, fibrillation very immediately threatens grave consequences to the patient""s well-being. It is imperative, therefore, that a defibrillation or other electronic system employed in such critical applications be particularly reliable in its operation. An individual""s very life may be hanging on the balance.
The switching function intrinsic to the proper operation of many electronic systems, generally, is especially important in applications such as a defibrillation system, wherein the timing of the signals that the system generates and/or applies remains so critical. This criticality results not only from the synchronizing function that the system serves, but also from the fact that the system""s vulnerability to potentially corrupting factors is often heightened during switching transient periods. Hence, it is essential for reliable and safe operation of many electronic systemsxe2x80x94not only that the switching of its operation occur promptly at the required points in timexe2x80x94but that the switching occur as quickly as possible. This would, among other things, minimize the device""s period of heightened vulnerability to failure.
An exemplary defibrillation system typically employs an H-bridge switching matrix to control the preparation and delivery of a prestored high voltage defibrillation pulse. The H-bridge includes a plurality of solid state switching devices, (typically four) that collectively provide the necessary switching actions. In many defibrillation system types, the solid state switching device is realized in the form of an insulated gate bipolar transistor (IGBT) for numerous reasons well recognized in the art.
To preserve reliability, measures are normally taken to electrically isolate the control terminals of switching devices from the actuating circuitry that generates the necessary control signals therefor. In certain applications, each solid state switching device is accordingly controlled by a signal prompted by the actuation circuitry through an electro-optical coupling. Devices such as photovoltaic isolators are employed in those cases, with the actuation circuitry driving one or more light emitting diodes in such manner that they optically excite one or more photovoltaic cells situated across an isolation gap from them. The photovoltaic cells responsively generate a control current that then charges the control terminal of the given switching device.
A number of practical problems are encountered when controlling the switching device in this manner. Perhaps the most prevalent of these problems is the insufficient rate at which the switching device""s control terminal is charged. With an IGBT or comparable device, for instance, the device capacitance seen at the control terminal, or gate, imposes a non-trivial charge time delay before the device assumes its fully saturated state; that is, where its emitter-collector conduction path approaches a short circuit condition. While sustained application of a constant current signal to the optically-exciting light emitting diodes of the photovoltaic isolator eventually yields sufficient charging of the switching device gate, the time delay incurred invariably results in a correspondingly delayed switching response. A practical implication of such delayed switching response is a prolonged period during which heightened levels of energy dissipation occur in the switching device. This requires of the device greater built-in tolerance measuresxe2x80x94measures which the implantability and/or other constraints pertaining to system applications such as defibrillators simply do not afford.
It is, therefore, particularly important in many applications that the period over which the device remains in the transitory state (between its xe2x80x98openxe2x80x99 and xe2x80x98closedxe2x80x99 circuit conditions) be minimized. During the transitory period, devices such as IGBTs operate within a characteristic active, or linear, region, wherein the device functions much like a resistor, dissipating substantial energy across its conduction terminals. Consequently, the device undergoes significant heating while it remains within this active region of operation. Permitting the device to remain in this region of operation over a prolonged duration permits a progressive accumulation of heat which heightens the risk of device failure, absent adequate means for dissipating the heat. Especially in implantable systems, highly restrictive size and weight constraints pose significant obstacles to providing adequate heat sinking capability in the device.
Optically-driven systems known in the art fail to provide a sufficient driving signal(s) to optimally minimize the transitory period for a given switching device operating under a given set of requirements. In a typical prior art optically-driven system for driving an IGBT switching device within a defibrillation system H-bridge, for example, full transition of the switching device occurs over no less than approximately 96 microseconds. The device undergoes progressive heating over much of that time; and, without measures in place specifically to curtail the period of heating, the device is exposed to a higher risk of failure than it needs to be.
In applications such as implantable defibrillators and other devices, another point of concern is the source by which a relay system switching device is driven. It is important to separately generate the energy necessary to drive the given switching device, apart from the generation of the actual energy to be delivered to the load in question. It is preferable that self-energizing control measures be employed accordingly to drive the switching device.
There is, therefore, a need for a relay system adapted to drive a load in highly responsive, yet highly reliable, manner. There is a need to realize such a system wherein adequate electrical isolation between at least a substantial portion of the given actuation circuitry and the given switching device(s) is effectively maintained, and wherein superfluous dissipation of energy is minimized.
2. Prior Art
Electro-optically driven relay systems for electrically driving a load are known in the art, as are such systems that employ photovoltaic isolator devices. The best prior art known to Applicant includes: U.S. Pat. Nos. 5,061,859; 5,329,210; 4,295,226; 5,132,553; 5,013,926; 4,902,901; 5,105,090; 4,777,387; 5,693,952; 4,723,312; 4,227,098; and, 4,390,790. The known systems disclosed in such prior art, however, fail to provide for the energization of one or more driver switching devices in the manner provided for by the subject driver system. They fail to provide the electro-optically coupled energization of a switching device with the degree of reliability attained by the subject relay system (for a given set of operational requirements).
For instance, U.S. Pat. No. 5,061,859 is directed to an optical isolator wherein a light emitting device (LED) is actuated to excite a photodiode that is connected to switching circuitry. The turn-on and turn-off speed of the LED is enhanced by control circuitry connected thereto which instantaneously augments the current flow at the rising and falling edges of an LED input signal. Prior to receiving this input signal, the LED is pre-biased by separate circuitry such that its capacitance is charged to a level just below that necessary to prompt its emission of light.
Unlike the subject relay system, however, the current augmentation in this prior art system is provided only instantaneously, and exclusively at only the rising and falling edges of the LED input signal. The momentary augmentation is generated by a pulse capacitor in the control circuitry and, therefore, is necessarily time-aligned with each input signal rising and falling edge. The augmentation pulse is not and cannot be applied substantially over a predetermined duration. Nor can the augmentation pulse be offset in time relative to the rising and falling edges of the LED input signal.
One implication of this is that the sum quantity of augmentation energy delivered to the LED within one LED input signal period is necessarily limited to a restrictive level. Indeed, compensatory measures in the form of the pre-bias circuitry attest to this fact. Another implication is that the quantity of energy yielded by the electro-optic coupling of the LED and photodiode may not be sufficient to actually energize a main switching device. The need to so energize a main switching device is altogether obviated in the disclosed system, for the signal derived via the electro-optic coupling is merely current flow through the photodiode that then initiates the operation of a pair of active amplifier circuits. It is the operation of these active circuits which ultimately energizes a voltage comparator that serves the main switching function.
This voltage comparator switching device is energized only upon prior operation of the active amplifier circuits connected thereto. The switching device cannot be energized directly by the electro-optically generated current signal.
It is therefore a primary object of the present invention to provide an electro-optically driven relay system that provides highly responsive and highly reliable operation.
It is another object of the present invention to provide an electro-optically driven relay system wherein the switching response of a switch unit is accelerated by augmenting an actuation signal over a predetermined duration.
It is yet another object of the present invention to provide an electro-optically driven relay system wherein a switch unit for selectively controlling energization of a load is rapidly energized for operation using an electro-optic current generating unit.
These and other objects are attained in an electro-optically driven relay system formed in accordance with the present invention. The subject relay system comprises: a switch unit for selectively controlling energization of a load responsive to a control signal; an electro-optic current generating unit connected to the switch unit for generating the necessary control signal responsive to an actuation signal; and, primary and supplemental drive units connected to the electro-optic current generating unit for collectively generating the actuation signal. The primary drive unit is operable responsive to a first input signal. The supplemental drive unit is operable responsive to a second input signal to augment the actuation signal for at least a predetermined non-zero duration. The switching response of the switch unit is thereby accelerated.
In one embodiment of the subject relay system, at least one supplemental drive unit is connected in a circuit path parallel to that in which the primary drive unit is connected, and the electro-optic current generating unit includes at least one photovoltaic isolator device. The supplemental drive unit in that embodiment is operable to augment the actuation signal by maintaining at least a predetermined current amplitude over at least the predetermined non-zero duration. Both the primary and supplemental drive units in that embodiment include a field effect transistor device, with the switch unit including at least one insulated gate bipolar transistor.